The previous article described the main technical classification of lead storage batteries, and here we mainly introduce the key technologies in advanced lead storage batteries: super battery and bipolar lead storage battery.
1. Super battery
The super battery is one of the latest improvements to lead storage batteries, originally developed for use in hybrid vehicles. Hybrid applications require the battery to be frequently charged and discharged at a high rate at a half state of charge (ie, SOC is 30% to 70%). This high-rate charge and discharge in a half-charged state will form a dense thin layer of lead sulfate on the surface of the negative electrode of the lead battery, which reduces the active surface area of the electrode, hinders further charging and discharging of the battery, and easily causes a hydrogen evolution reaction on the negative electrode, which greatly reduces the battery life. This situation is also known as the phenomenon of “sulfation of the negative electrode”, which is an important failure mode of lead storage batteries.
The supercapacitor is used in parallel with the lead storage battery, and the frequent high-rate charge and discharge current is mainly provided by the supercapacitor, which can effectively prolong the service life of the lead storage battery. However, since the working voltage characteristics of supercapacitors are very different from those of lead storage batteries, a relatively complex parallel control circuit is required externally, which not only increases the complexity of the system, but also increases the cost.
Japan’s Furukawa Battery Company and Australia’s CSIRO National Laboratory jointly developed a new lead storage battery technology in 2005-2006, and named it super battery (ie ultrabattery technology). The principle is to integrate the supercapacitor into the internal structure of the lead storage battery without external circuit control, which not only improves the power density and service life of the battery, but also maintains the characteristics of low cost. In 2008, EastPenn of the United States was authorized by Furukawa to produce and sell super batteries in the Americas.
The structural principle of the super battery is shown in Figure 1.
In Figure 1, the upper left is a lead storage battery, which consists of a PbO2 positive electrode and a sponge Pb negative electrode; the upper right is an asymmetrical supercapacitor of a lead storage battery system, which consists of a PbO2 positive electrode and a supercapacitor activated carbon negative electrode. Combining these two structures forms the super battery below. Inside the super battery, the lead negative electrode of the lead battery and the super capacitor activated carbon negative electrode are directly connected in parallel, sharing the PbO2 positive electrode. When the super battery is working, the charging and discharging current is jointly provided by the lead battery and the super capacitor. Since the internal resistance of the supercapacitor is much smaller than that of the lead storage battery, the larger current is provided by the supercapacitor during operation. The supercapacitor acts as a current buffer for the Pb negative electrode to protect the Pb negative electrode from being damaged by excessive charge and discharge currents, greatly extending its life.
The working principle of the asymmetric supercapacitor integrated inside the super battery is shown in Figure 2.
In Figure 2, the positive electrode is the positive electrode of the lead storage battery, so it is the same as the lead storage battery during charging and discharging, that is, the conversion of PbO2 and PbSO4. The negative electrode is an activated carbon electrode with a high specific surface area. When charging, an adsorption layer of hydrogen ions and electrons will be formed on the surface of the activated carbon electrode, forming a layer of capacitance to store energy. During discharge, hydrogen ions desorb and electrons are released at the same time.
2. Bipolar lead acid battery
Bipolar lead acid battery is a lead battery made of bipolar plates, which refers to a conductive substrate, with a negative active material on one side and a positive active material on the other, and then assembled from these bipolar plates and separators.
The first bipolar lead acid battery was made by Kapitza in England in 1923. By the 1960s, after many attempts, bipolar lead acid batteries with various uses were developed, including hybrid electric vehicle batteries, automobile starting batteries, and naval sonar batteries. However, these batteries are flooded and use bulky lead plates, thus failing to achieve commercial mass production. In 1985, Rowlette in the United States combined the bipolar lead acid battery with the valve-regulated lead storage battery structure developed by Gate, and developed the first bipolar sealed lead acid battery, making it a big step toward practical application.
The traditional lead storage battery is shown in Figure 3, which consists of a series of independent positive plates and negative plates connected in series and parallel to obtain the required voltage capacity. The cells of each single cell in the battery pack are isolated from each other by the partition wall in the middle of the battery slot, and are connected in series through the connecting bars between the cells to achieve a certain voltage. The current is conducted along the grid, through the connecting bars between the tabs and the cells, and finally to the electrodes. This is a long path for the current, and part of the energy is inevitably consumed in the middle, resulting in a reduction in the final output energy.
The lead storage battery shown in Figure 4 is a 6V battery pack consisting of 3 single cells. The 2 bipolar electrodes separate the battery into 3 cells and provide electrical circuits between the positive and negative active materials of adjacent cells while the bipolar battery pack must have two monopolar cells. That is, a unipolar positive electrode with positive active material connected to the positive terminal and a unipolar negative electrode with negative active material connected to the negative terminal.